Frac water heater and fuel oil heating system

ABSTRACT

An accessory heat exchanger unit for heating the fuel oil used by the frac oil heater system of the invention prior to combustion. The accessory heat exchanger unit receives and distributes the fuel oil through a plurality of heat exchanger tubes prior to directing the oil to the combustion chamber. The unit also includes an inlet for receiving heated treatment fluid from the frac water heater system of the present invention. The heated treatment fluid passes over the plurality of heat exchanger tubes of the accessory heat exchanger unit prior to being directed to the outlet of the accessory heat exchanger unit. By heating the fuel oil prior to combustion, the accessory heat exchanger unit greatly improves the viscosity and flow rate of the fuel oil, thereby allowing cheaper, less cold-tolerant grades of fuel oil to be used in extreme cold climates with no degradation in the operation of the frac water heater system of the present invention.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/352,505 filed on Jan. 12, 2009, which claims the benefit ofand priority to U.S. Provisional Patent Application No. 61/078,734 filedJul. 7, 2008, the technical disclosure of which is hereby incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to an apparatus and method for heatingfuel oil prior to combustion for subsequent use in heating a water orpetroleum based fluid for injection into an oil or gas well or into apipeline system. In particular, the present invention relates to anapparatus and method for pre-heating fuel oil prior to combustion toimprove its viscosity and combustion characteristics in cold weather.

2. Description of the Related Art

It is common in the oil and gas industry to treat oil and gas wells andpipelines with heated fluids such as water and oil. For example, onesuch application commonly known as a hydraulic fracturing job or “frac”job, involves injecting large quantities of a heated aqueous solutioninto a subterranean formation to hydraulically fracture it. Such fracjobs are typically used to initiate production in low-permeabilityreservoirs and/or re-stimulate production in older producing wells.Water is typically heated to a specific temperature range to preventexpansion or contraction of the downhole well casing. The heated wateris typically combined with a mixture of chemical additives (e.g.,friction reducer polymers which reduce the viscosity of the water andimprove its flowability so that it's easier to pump down the well),proppants (e.g., a special grade of light sand), and a cross-linked guargel that helps to carry the sand down into the well. This fraccing fluidis then injected into a well hole at a high flow rate and pressure tobreak up the formation, increasing the permeability of the rock andhelping the gas or oil flow toward the surface. As the fraccing solutioncracks the rock formation, it deposits the sand. As the fractures try toclose, the sand keeps them propped open. Frac jobs are typicallyperformed once when a well is newly drilled, and again after a couple ofyears when the production flow rate begins to decline

Another application, commonly referred to as a “hot oil treatment”,involves treating tubulars of an oil and gas well or pipeline byflushing them with a heated solution to remove build up of paraffinalong the tubulars that precipitate from the oil stream that is normallypumped therethrough.

Frac jobs and hot oil treatments are typically performed at the remotewell sites and usually require less than a week to complete.Consequently, the construction of a permanent heating facility at thewell site is not cost effective. Instead, portable heat exchangers,which are capable of transport to remote well sites via improved andunimproved roads, are commonly used.

In the past, such portable heat exchangers have typically employedgas-fired heat sources using a liquefied petroleum gas (LPG) such aspropane to heat treatment fluids at remote well sites. Such gas-firedheater units typically include a tubular coil heat exchanger configuredabove one or more open flame gas burners in an open-ended fireboxhousing. The tubular coil heat exchanger typically comprises a fluidinlet in communication with a plurality of interconnected tubes, whichin turn communicate with a fluid outlet. The plurality of tubes aretypically arranged in a stacked configuration of planar rows, whereineach tube in a row is aligned in parallel with the other tubes. Theoutlet of each tube is connected in series to the inlet of an adjacenttube in the row by means of a curved tube or return bend. Similarly,each planar row is connected to the adjacent rows above and below byconnecting the outlet of the outermost tube in one row with the inlet ofthe outermost tube in another row by means of a curved tube or returnbend.

The one or more gas burners are typically positioned below the tubularcoil heat exchanger so as to project a vertical flame up and through theheat exchanger. The gas burners are supplied with gas fuel from a nearbygas storage tank (e.g., a propane tank). Ambient air is also supplied tothe burners via the opened-ended bottom of the firebox housing. The hotflue gasses generated from the burning of the LPG rise up and throughthe tubular coil heat exchanger within the firebox housing and exhaustvia a vent at the top of the firebox housing.

While gas-fired heat sources are adequate for performing many oil fieldservicing tasks, they exhibit a number of inherent drawbacks. Theseinherent limitations significantly impact their effectiveness inperforming certain heating operations at remote oil field work sites.For example, frac jobs typically require the production of massivevolumes of heated water. While gas-fired heat sources are certainlycapable of heating fluids such as water, they are poorly suited toheating in a timely manner large volumes of continuously flowing waterin many commonly occurring climactic and atmospheric conditions.Moreover, the logistics involved in conducting such heating operationsat remote work sites negatively impacts the cost efficiencies of such asystem.

For example, LPG (e.g., propane gas) has a relatively low energy contentand density when compared to other fuel options. For example, dieselfuel when properly combusted typically releases about 138,700 Britishthermal units (BTU) per US gallon, while propane typically releases onlyabout 91,600 BTU per liquid gallon, or over 33% less. Thus, gas-firedheating units often lack sufficient heating capacity to producesufficient quantities of heated water rapidly enough for the requiredoperation to be completed. Consequently, in order to provide sufficientquantities of heated water on a timely basis for a typical frac job, thetreatment water must often be preheated and stockpiled in numerous fracwater holding tanks. These holding tanks range in size up to 500 bbl(i.e., approximately 21,000 gallons). It is not unusual for a typicalfrac job to require 10 or even 20 frac water holding tanks at the remotework site. The preheated water is typically overheated so as to allowfor cooling while waiting to be injected into the well. Oftentimes, thepreheated treatment water must be reheated just prior to injection intothe well head. Needless to say, the logistics involved with providingadditional holding tanks at the remote work site and the additionalcosts incurred in overheating or reheating the supply water negativelyimpacts the efficiency of the overall operation.

While the technique of overheating and stockpiling supply water canameliorate some the shortcomings in the heating capacity of gas-firedheat sources, in certain circumstances (e.g., severely cold weather orhigh altitude) it is inadequate. This is due to a number of reasons.First, the temperature change requirement for the system is simplygreater in colder weather. That is, in colder weather the intake watersupplied to the gas-fired heating unit is colder while the requiredinjection temperature remains essentially the same. Thus, it takeslonger for the gas-fired heating unit to preheat the supply water. Theproblem is further compounded by the fact that the stockpiled preheatedwater cools more rapidly in colder weather. Moreover, at higheraltitudes there is less oxygen in the ambient atmosphere for combustionin the gas burner. Thus, at higher altitudes the heating capacity ofgas-fired heat sources is further reduced.

In addition, propane gas requires large and heavy high-pressure fueltanks for its transport to remote sites. The size of such high-pressurefuel tanks is, of course, limited by the size of existing roads. Thus, atypical frac job may require the transport of multiple largehigh-pressure fuel tanks to a remote site to ensure an adequate supplyof fuel to complete the operation.

Furthermore, there are several safety concerns which must be taken intoconsideration when using gas-fired heat sources. As mentionedpreviously, current gas-fired heat exchangers typically use an openflame burner, i.e., a burner which is open to the ambient atmosphere.The fire boxes of such heat exchanger are typically elevated above theground and opened on the bottom. The gas-fired burners are typicallypositioned near the open bottom of the firebox and directly below theheat exchange tubing. The gas-fired burners draw ambient air asnecessary to assist in the combustion of the propane gas. While simpleand efficient in providing air for combustion, open flame burnerspresent a number of safety concerns. An open flame at the well siteposes a substantial risk of explosion and uncontrolled fire, which candestroy the investment in the rig and injure or even cost the lives ofthe well operators. Moreover, open flame burners are particularlysusceptible to erratic burning or complete blow-out in gusty windconditions. Current U.S. government safety regulations provide that theopen flame heating of the treatment fluids cannot take place within theimmediate vicinity of the well.

While safety concerns are of overriding importance, compliance with theno open-flame regulations requires additional time and expense toconduct heated fluid well treatments. Thus, there has been a long feltneed for a safer and more efficient apparatus and method of heating atreatment fluid for injecting into the tubulars of oil and gas wells andpipelines without using an open flame heat source in the vicinity of thetreatment location.

The present invention discloses an oil-fired heat exchanger, whichprovides enhanced frac-water heating capabilities in the field. However,operation of the present invention in extreme cold climates has revealeda need for additional improvements. For example, in extreme coldclimates standard fuel oil (e.g., standard diesel fuel) used forcombustion by the system tends to gel together. Moreover, paraffin (acommon component of standard diesel fuel) begins to crystallize whencold weather strikes, which further degrades the viscosity and flowcharacteristics of the fuel oil in the system of the present invention.Maintaining proper viscosity and flow characteristics of the fuel oil inthe system of the present invention is critical to ensuring propercombustion of the air-fuel mixture. In the past, this problem has beenalleviated by using fuel additives or switching to a more cold-tolerantgrade of fuel oil when operating in extreme cold climates. However, suchremedies often come with significant increases in operating costs. Forexample, using fuel additives or switching to more cold-tolerant gradesof fuel oil noticeably increases the operating costs on a per gallonbasis. When multiplied by the large volumes consumed by the frac-waterheater of the present invention, the costs associated with an operatingcycle are significant. For example, the frac-water heater of the presentinvention can easily consume well over 8,500 gallons of fuel oil in a24-hour operation. Thus, a further need exists for an improved methodand apparatus for operating the present frac-water heating system inextreme cold climates without the added expense of fuel additives orusing a more cold-tolerant grade of fuel oil.

SUMMARY OF THE INVENTION

The present invention overcomes many of the disadvantages of prior artmobile oil field heat exchange systems by providing an oil-fired heatexchange system. The present invention is a self-contained unit which iseasily transported to remote locations. In one embodiment, the presentinvention is disposed on a trailer rig and includes a closed-bottomfirebox having a forced-air combustion and cooling system. The rig alsoincludes integral fuel tanks, hydraulic and pneumatic systems foroperating the rig at remote operations in all weather environments. In apreferred embodiment, the oil-fired heat exchanger system is used toheat water on-the-fly (i.e., directly from the supply source to the wellhead) to complete a hydraulic fracturing operation.

The present invention comprises a closed firebox that includes a novelheat exchanger comprised of a single-pass tubular coil configured in ahighly oscillating or serpentine manner and oriented along multiple axesso as to maximize its exposure to the heat generated by the oil-firedburner assemblies. The design of the heat exchanger includes ahorizontal tunnel configured within a bottom portion. The oil-firedburner assemblies are configured and oriented in relation to the tunnelso that their flames are initially generated in a horizontal fashioninto the tunnel within the heat exchanger.

The present invention further includes a novel forced-air combustion andcooling system. The forced-air system is comprised of a primary airsystem and a secondary air system. The primary air system providespressurized air directly to the oil-fired burner assemblies to maximizeatomization and combustion of the fuel oil. The secondary air systemprovides pressurized air to strategic positions within the firebox toassist in controlling the cooling of the firebox and to maximize thecombustion of the fuel/air mixture. In addition, vents and ventpassageways are formed in the wall of the firebox and supplysupplemental ambient air to front of the burner assemblies. The ventsallow the system to be operated more safely by allowing burner accessdoors to be configured in a “down” or “closed” position duringoperations, which significantly reduces the operational noise created byburner assemblies when operating. Moreover, with the burner access doorsconfigured in the “down” position, the danger inherent in a blowbackevent of the burner assemblies is greatly reduced. The primary andsecondary air systems are powered by hydraulic pumps integral to theoverall system. The present invention also includes systems forregulating and adjusting the fuel/air mixture within the firebox tomaximize the combustion efficiency.

The improved system of the present invention also includes severalsubsystems for maximizing the safety and efficiency of the heatexchanger system. The system includes a novel hood mechanism attached tothe exhaust stack of the firebox. In addition, the system includes anovel intake air muffler/silencer system, which significantly reducesthe noise generated by the intake of such large quantities of ambientair.

The system also includes novel methods for heating large volumes oftreatment fluids, such as water, in a continuously flowing fashion sothat heating operations can be performed “on-the-fly”, i.e., without theuse of preheated stockpiles of treatment fluid. For example, water atambient conditions can be drawn into the device of the present inventionand heated so that sufficient volumes of continuously flowing heatedtreatment fluid may be supplied directly to the well head for conductinghydraulic fracturing operations on the well. The system also includesnovel methods for controlling the heating of the treatment fluid as itpasses through the system. The system further includes novel methods forcontrolling the temperature change and volume flow of treatment fluid asit passes through the system.

The improved system of the present invention may further include anaccessory fuel oil heating system for heating the fuel oil prior tocombustion. The accessory fuel oil heating system includes a heatexchanger unit having a manifold for receiving and distributing the fueloil through a plurality of heat exchanger tubes. The unit furtherincludes an inlet for receiving heated treatment fluid from the fracwater heater system of the present invention. The heated treatment fluidfrom the frac water heater system passes over the plurality of heatexchanger tubes of the accessory heat exchanger unit prior to beingdirected to the outlet of the accessory heat exchanger unit. Theaccessory fuel oil heating system can be integral to the frac waterheater system of the present invention or comprise a separate unit whichmay be selectively incorporated into an oil-fired frac water heatingsystem. By heating the fuel oil prior to combustion, the accessory fueloil heating system of the present invention greatly improves theviscosity and flow rate of the fuel oil, thereby allowing cheaper, lesscold-tolerant grades of fuel oil to be used in extreme cold climateswith no degradation in the operation of the system of the presentinvention. In addition, any preheated fuel oil that is not sent to theburner will be returned to the fuel tank to help preheat the fuel of inthe fuel tank.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the method and apparatus of the presentinvention may be had by reference to the following detailed descriptionwhen taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a perspective view of an embodiment of the Oil-Fired HeatExchanger of the present invention;

FIG. 2A is a left side elevation view of the embodiment of the Oil-FiredHeat Exchanger of the present invention shown in FIG. 1;

FIG. 2B is a right side elevation view of the embodiment of theOil-Fired Heat Exchanger of the present invention shown in FIG. 1;

FIG. 2C is a close-up view of the mechanism for opening and closing theopposing hood doors of the embodiment of the Oil-Fired Heat Exchanger ofthe present invention shown in FIG. 2B;

FIG. 3 is an overhead plan view of the embodiment of the Oil-Fired HeatExchanger of the present invention shown in FIG. 1;

FIG. 4A is a front perspective view of an embodiment of the heatexchanger of the Oil-Fired Heat Exchanger of the present invention;

FIG. 4B is a back perspective view of the embodiment of the heatexchanger shown in FIG. 4A;

FIG. 4C is a cross-sectional view of the embodiment of the heatexchanger shown in FIGS. 4A and 4B installed in the embodiment of theOil-Fired Heat Exchanger of the present invention shown in FIG. 1;

FIG. 5 is perspective view of a portion of the primary and secondary airsystems of the Oil-Fired Heat Exchanger of the present invention;

FIG. 6 is cut-away cross-sectional view of a portion of the secondaryblower section of the secondary air system of the Oil-Fired HeatExchanger of the present invention;

FIG. 7 is a schematic depiction of the hydraulic, fuel, and air supplysystems of the embodiment of the Oil-Fired Heat Exchanger of the presentinvention shown in FIG. 1;

FIG. 8 is an overhead view of the schematic depiction of the hydraulic,fuel, and air supply systems of the embodiment of the Oil-Fired HeatExchanger of the present invention shown in FIG. 7;

FIG. 9 is a schematic depiction of an embodiment of the accessory fueloil heating system of the present invention;

FIG. 10 is a perspective view of an embodiment of an accessory heatexchanger unit in the system thereof’

FIG. 11A is an overhead plan view thereof;

FIG. 11B is a partial cross-sectional view thereof;

FIG. 12 is a partial exploded view thereof;

FIG. 13A is an elevation view of the inlet end thereof;

FIG. 13B is a partial cross-sectional view of the inlet end shown inFIG. 13A;

FIG. 14A is a cross-sectional view of the accessory heat exchanger unitshown in FIG. 11B at line 14A-14A; and

FIG. 14B is a plan view of the support bracket shown in FIG. 14A.

Where used in the various figures of the drawing, the same numeralsdesignate the same or similar parts. Furthermore, when the terms “top,”“bottom,” “first,” “second,” “upper,” “lower,” “height,” “width,”“length,” “end,” “side,” “horizontal,” “vertical,” and similar terms areused herein, it should be understood that these terms have referenceonly to the structure shown in the drawing and are utilized only tofacilitate describing the invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the Figures, and in particular to FIGS. 1 and 2A-C, anembodiment of the improved oil-fired heat exchanger system 100 of thepresent invention is shown. The embodiment 100 shown in the Figures isconfigured to be an oil-fired frac water heater system. As depicted, theembodiment of the frac water heater system 100 is configured on a dropdeck trailer 14 and suitable for transport to remote oil field sites.The system 100 includes a fuel storage and supply system, a firebox 40containing a single heat exchanger 50, primary 70 and secondary 80 airsupply systems connected to the firebox 40, and an auxiliary power plant30 for driving an accessory gearbox 32. The accessory gearbox 32, inturn, drives multiple hydraulic pumps, which power a main fluid pump 94and the air supply systems. The main fluid pump 94 is used to drawfluid, such as water, from a fluid source and supply it to the intake 51of the heat exchanger 50. The hydraulic pressure generated by the mainfluid pump 94 effectively pumps the fluid through the heat exchanger 50where it is heated. As the treatment fluid proceeds through a singlepass of the heat exchanger 50 it increases in temperature until itreaches an outlet 52 of the heat exchanger 50 where it is directed viatubular conduits or hose to the well head for injection into theformation. The system 100 also includes a control quadrant 10 andcontrol levers 12 for operating and monitoring the system 100.

As shown in the embodiment depicted in the Figures, the entire fracwater heater system 100 is configured on a single drop deck trailer 14having multiple wheels 16 and connected to a separate towing vehicle 2.It is understood that alternate embodiments of the system of the presentinvention may be skid mounted or configured integral to a singlevehicle. In addition, the subject invention may also be configured sothat one or more of the various components of the system (e.g., fueltank 20, firebox 40, and auxiliary power plant 30) are configured onseparate trailers, vehicles or skids for transport to the remote worksite.

With reference again to the Figures, and in particular to FIGS. 2A-2Cand 3, the components of the embodiment of the improved oil-fired heatexchanger system 100 of the present invention will be described ingreater detail. As depicted in the Figures, the embodiment the presentinvention 100 is disposed on a single trailer rig 14 and includes afirebox 40 containing a single heat exchanger 50, primary 70 andsecondary 80 air supply systems connected to the firebox 40, a fuelsystem for storing and supplying fuel to multiple burner assemblies 60configured in the firebox 40, and an auxiliary power plant 30, whichpowers multiple hydraulic systems and assorted auxiliary systems. Thefirebox 40 may also include at least one vent 40 b and passageway 40 cwhich supplies ambient air from the upper exterior of the firebox 40 tothe front of the burner assemblies 60. The vent 40 b and passageway 40 cenable the burner assemblies 60 to operate with the access door 40 aconfigured in a “closed” position, which significantly reduces theoperational noise created by burner assemblies 60 when operating.Moreover, with the burner access doors 40 a configured in the “down”position (shown in FIG. 1), the danger inherent in a blowback event ofthe burner assemblies 60 is greatly reduced.

Auxiliary Power Plant & Hydraulic System

As depicted in the Figures, the auxiliary power plant 30 is configurednear the front end of the trailer 14. The auxiliary power plant 30provides power for driving an accessory gearbox 32 and assortedauxiliary systems (e.g., electric, pneumatic). In one embodiment, theauxiliary power plant 30 comprises a diesel engine, which includes anelectric alternator and air compressor. Alternatively, the electricalternator and air compressor may be powered by the accessory gearbox32. The electric alternator provides electrical power to the system 100and the pneumatic compressor provides pneumatic pressure for controllingthe system 100.

The auxiliary power plant 30 provides the primary motive force fordriving the accessory gearbox 32. The accessory gearbox 32, in turn,drives multiple hydraulic pumps that power the hydraulic systems of thepresent invention. Each hydraulic pump is used to power an independenthydraulic circuit. For example, in the depicted embodiment, theaccessory gearbox 32 powers three hydraulic circuit systems. The firsthydraulic circuit includes a first hydraulic pump 33 that suppliespressurized hydraulic fluid via supply/return line 33 a to a firsthydraulic motor 36, which powers the first air blower system. The secondhydraulic circuit includes a second hydraulic pump 34 that suppliespressurized hydraulic fluid via supply/return line 34 a to the secondhydraulic motor 37, which powers the second air blower system. The thirdhydraulic circuit includes a third hydraulic motor 35 that suppliespressurized hydraulic fluid via supply/return line 35 a to a thirdhydraulic motor 38, which powers the main fluid pump 94. The threehydraulic systems are supplied by a hydraulic reservoir 31 positionednear the accessory gearbox 32. In a preferred embodiment, the threehydraulic pumps 33, 34, each comprise a mechanically-driven,variable-displacement, hydraulic pump; while the three hydraulic motors36, 37, 38 each comprise fixed displacement hydraulic motors. Thehydraulic pumps 33, 34, 35 are rated at 5000 psi, but typically operatedat approximately 2500-3000 psi.

Treatment Fluid Supply System

The main fluid pump 94 is used to draw a treatment fluid, such as water,from a fluid source and supply it to the inlet 51 of the heat exchanger50. The main fluid pump 94 is typically integral to the system 100 andhas sufficient power to both draw the treatment fluid from a source andto pump the treatment fluid through the heat exchanger 50 and on to thewell head for subsequent injection into the formation. In oneembodiment, the main fluid pump 94 comprises a hydraulically-poweredcentrifugal fluid pump that is capable of supplying treatment fluid tothe heat exchanger 50 at a pressure of about 150 psi. The volume oftreatment fluid pumped through the heat exchanger 50 will vary with thepump speed. In a preferred embodiment, the main fluid pump 94 is capableof pumping a maximum of 252 gpm of treatment fluid through the heatexchanger 50.

As shown in the Figures, the fluid supply system may include an intake90 manifold for connecting one or more supply hose (not shown) to thesystem's respective intake. The intake manifold 90 may include one ormore spigots 91 for receiving supply hose in fluid communication withthe fluid source. Each inlet spigot 91 may further include a valvemechanism 92, which selectively controls the fluid flow through itsrespective inlet spigot 91. Tubular intake conduits 93 a, 93 b fluidlyconnect the inlet of the main fluid pump 94 with the intake manifold 90.Conduit 93 c fluidly connects the outlet of the main fluid pump 94 withthe inlet 51 of the heat exchanger 50. The hydraulic pressure generatedby the main fluid pump 94 effectively pumps the fluid through the heatexchanger 50 where it is heated. As the treatment fluid proceeds througha single pass of the heat exchanger 50 it increases in temperature untilit reaches an outlet 52 of the heat exchanger 50 where it is directedvia tubular outlet conduit 95 and supply hose (not shown) to the wellhead for injection into the formation. As shown in the Figures, thefluid supply system may further include an outlet manifold 96 having oneor more spigots 97 for connecting with supply hose. Each outlet spigot97 may further include a valve mechanism 98, which selectively controlsthe fluid flow through its respective outlet spigot 97.

Fuel Supply & Control System

As shown in the Figures and schematically depicted in FIGS. 7 and 8, thefuel system includes a fuel tank 20, which is configured near the rearor back end of the trailer 14. The fuel tank 20 is typicallyunpressurized and used to store the liquid fuel used by the multipleburner assemblies 60 configured in the firebox 40. In the depictedembodiment 100, the fuel tank 20 is unpressurized and can hold up to 60bbl of diesel fuel. The fuel system also includes an unpressurized fuelline 21, which supplies fuel from the fuel tank 20 to the intake of afuel pump 22. The fuel pump 22 boosts the fuel pressure and directs itto the multiple burner assemblies 60 by means of a pressurized fuel line26. In one embodiment, the fuel pump 22 boosts the fuel pressure toapproximately 50-100 psi, preferably 60 psi.

The fuel system also includes a pressure relief valve 24 in fluidcommunication with the pressurized fuel line 26. The pressure reliefvalve 24 permits fuel to vent back into the fuel tank by means of fuelline 25 when the fuel pressure in the pressurized fuel line 26 exceeds acertain pressure.

The fuel system further includes a fuel pressure control motor valve 27,which regulates the flow of fuel from the pressurized fuel line 26. Thepressurized fuel line 26 fluidly connects the outlet of the fuel pump 22with the inlet of a fuel pressure control motor valve 27. The fuelpressure control motor valve 27 controls the amount of fuel supplied tothe multiple burner assemblies 60 via pressurized metered fuel lines 28.As depicted in the drawings, the metered fuel lines 28 may be configuredso as to supply pressurized fuel to sets of burner assemblies, which arecomprised of more than one burner assembly 60. The fuel pressure controlmotor valve 27 may be electrically, pneumatically or hydraulicallyactuated. In a preferred embodiment, the fuel pressure control motorvalve 27 comprises a pneumatically-actuated flow control valve.

The temperature of the treatment fluid exiting the heat exchanger outlet52 is a function of three variables: the volumetric flow rate of thetreatment fluid through the heat exchanger 50; the flow rate of thepressurized secondary air; and the heat generated by the multiple burnerassemblies 60 configured in the heat exchanger 50. The flow rate of thesecondary air is typically held constant during all operations while thevolumetric flow rate of the treatment fluid is typically constant for agiven operation. Thus, the temperature of the treatment fluid exitingthe heat exchanger outlet 52 is controlled by regulating the volume offuel supplied to the multiple burner assemblies 60.

An adjustable temperature controller mechanism 68 is used to send acontrol signal, which causes the fuel pressure control motor valve 27 toopen or close, thereby increasing or decreasing the volume of fuelsupplied to the multiple burner assemblies 60 via pressurized meteredfuel lines 28. The control signal may comprise an electrical, wireless,pneumatic, or hydraulic signal. For example, in one embodiment, theadjustable temperature controller mechanism 68 comprises a simple manualrotary or slider rheostat device, which controls an electric signal thatcontrols the actuation of the fuel pressure control motor valve 27. Inanother embodiment, the adjustable temperature controller mechanism 68comprises a simple manual rotary valve, which controls a pneumaticpressure signal that controls the actuation of the fuel pressure controlmotor valve 27.

The temperature controller mechanism 68 may further includes athermostat mechanism, which continually monitors the temperature of thetreatment fluid exiting the heat exchanger outlet 52 and automaticallyadjusts the control signal to the fuel pressure control motor valve 27to open or close as necessary to maintain a set point temperature.

Thus, the fuel pressure supplied to the multiple burner assemblies 60 isinitially generated by the fuel pump 22 and regulated by the fuelpressure control motor valve 27. For example, in the previously notedembodiment, the fuel pump 22 boosts the fuel pressure to approximately50-100 psi, preferably 60 psi. The fuel pressure is limited to a maximumpressure of 100 psi by the pressure relief valve 24, which permits fuelto vent back into the fuel tank by means of fuel line 25 when the fuelpressure in the pressurized fuel line 26 exceeds 100 psi. The fuelpressure control motor valve 27 regulates the maximum fuel pressuresupplied to the multiple burner assemblies 60 via pressurized meteredfuel lines 28 to approximately 60 psi.

Firebox

As depicted in the Figures, the firebox 40 is configured near the centerof the trailer 14. The firebox 40 is a closed-bottomed box having one ormore exhaust stacks 42 configured near the top. In a preferredembodiment, the outer shell of the firebox 40 is constructedsubstantially of 3/16″ carbon steel. The firebox 40 houses a single heatexchanger 50 and a plurality of burner assemblies 60 for heating atreatment fluid during a single pass through the heat exchanger 50. Theclosed-bottom design of the firebox 40 ensures the plurality of burnerassemblies 60 are less susceptible to changes in ambient conditions,such as wind direction or gustiness. The interior walls and bottom ofthe firebox 40 are lined with an insulating refractory material. Therefractive lining 48 is configured between the interior walls and bottomof the firebox 40 and the heat exchanger 50. In one embodiment, therefractive lining 48 comprises one or more layers of fiber-typeinsulation coated with a cementious refractive compound. In a preferredembodiment, the firebox 40 further includes at least one vent 40 b andpassageway 40 c, which supplies ambient air from the upper exterior ofthe firebox 40 to the front of the burner assemblies 60. The vent 40 band passageway 40 c enable the burner assemblies 60 to operate with theaccess door 40 a configured in a “closed” position, which significantlyreduces the operational noise created by burner assemblies 60 whenoperating. Moreover, with the burner access doors 40 a configured in the“down” position, the danger inherent in a blowback event of the burnerassemblies 60 is greatly reduced.

Exhaust Stacks

As previously noted, one or more exhaust stacks 42 are configured nearthe top the firebox 40 providing an exhaust for flue gases to exit thefirebox 40. In the depicted embodiment, the firebox 40 further includesa tapered hood assembly 41, which incorporates the one or more exhauststacks 42. The tapered hood assembly 41 is removable so as to allowaccess to the heat exchanger 50 for servicing. Each exhaust stack 42also includes a hood door assembly 44, which is opened when the system100 is operating. As depicted in FIG. 2A, each hood door assembly 44includes two doors 44 a, 44 b which are pivotally mounted to opposingsides of a respective exhaust stack 42.

Hood Door Opening Mechanism

With reference to FIG. 2B, each hood door assembly 44 may furtherinclude a novel mechanism 46 for opening and closing the opposing hooddoors. As shown in greater detail in FIG. 2C, the mechanism 46 comprisesa series of bell crank mechanisms, which cause the hood doors to open orclose when actuated. The embodiment in FIG. 2C depicts the hood doorassembly 44 on the left side in an opened position and the hood doorassembly 44 on the right side in a closed position. Each mechanism 46comprises a piston 46 a having one end attached to the firebox 40 and asecond end attached to a first bell crank 46 b. The first bell crank ispivotally attached to the side of the firebox 40. When actuated, thepiston 46 a causes the first bell crank 46 b to rotate about its pivotpoint p₁. The first bell crank 46 b also includes a pivotally attachedpush rod linkage 46 c that connects the first bell crank 46 b to asecond bell crank 46 d, which is fixably attached to the side edge ofone of the hood doors 44 a. The second bell crank 46 d is configured sothat its pivot point p₂ is co-aligned with that of its respective hooddoor. The second bell crank 46 d also includes a pivotally attached pushrod linkage 46 e that connects the second bell crank 46 d to a thirdbell crank 46 f, which is also fixably attached to the side edge of theother of the hood doors 44 b. The third bell crank 46 f is alsoconfigured so that its pivot point p₃ is co-aligned with that of itsrespective hood door. Actuating the piston 46 a causes the extension orretraction of a piston rod r_(p), which causes each of the three bellcranks to rotate simultaneously about their respective pivot points.This, in turn, causes the hood doors 44 a, 44 b to pivot open or closedas desired. In a preferred embodiment, the piston 46 a is apneumatically actuated piston.

Burner Assemblies

The firebox 40 also includes a plurality of burner assemblies 60, whichare configured in the lower side of the firebox 40. As will besubsequently described in greater detail, each of the burner assemblies60 are connected to the fuel system and a pressurized air supply. Forexample, as schematically depicted in FIGS. 7 and 8, liquid fuel issupplied to each burner assembly 60 via the metered pressurized fuelline 28. Similarly, pressurized air for combustion is supplied to eachburner assembly 60 via a primary air conduit 78 c. The pressurized airand fuel are combined in the burner assembly 60 and directed through anatomizer nozzle 64, which projects an atomized fuel spay into thefirebox 40 where it is combusted. Each burner assembly 60 is configuredin the lower side of firebox 40 so as to initially generate asubstantially horizontal combustion flow within the firebox 40. Eachburner assembly 60 includes self-contained controls for adjusting thefuel-air mixture and an ignition mechanism for initially igniting thefuel-air mixture. In a preferred embodiment, the burner assembly 60comprises a 780-Series self-proportioning, oil-fired burner manufacturedby the Hauck Manufacturing Company of Lebanon, Pa.

Heat Exchanger

The heat exchanger 50 contained within firebox 40 is comprised of atubular coil which is configured in a highly oscillating or serpentinemanner and oriented along multiple axes so as to maximize its exposureto the heat generated by the oil-fired burner assemblies 60. The heatexchanger coil 50 includes a single inlet 51 configured at or near thetop of the heat exchanger coil 50 and a single outlet 52 configured ator near the bottom of the heat exchanger coil 50. Such a configurationgreatly improves the efficiency of the system 100 by minimizing the backpressure exerted on the main fluid pump 94 by the treatment fluid andproviding a gravity assist to the flow of treatment fluid through theheat exchanger 50. As the treatment fluid proceeds through a single passthrough of the heat exchanger coil 50 it increases in temperature untilit reaches the outlet 52 where it is directed, via an outlet conduit 95and supply hose (not shown), to the well head for injection into theformation.

With reference now to FIGS. 4A-4B, an embodiment of the heat exchanger50 of the present invention is depicted. The heat exchanger 50 iscomprised of a tubular coil which is configured in a highly oscillatingand serpentine manner and oriented along two axes so as to maximize itsexposure to the heat generated by the oil-fired burner assemblies 60.For example, the depicted embodiment of heat exchanger 50 includes anupper portion 53 configured in stacked horizontal rows of tubing fakeddown in a series of reversing loops oriented about a vertical axis; anda lower portion 56 configured in a helical coil oriented about ahorizontal axis. The upper portion 53 is fluidly connected to the lowerportion 56 forming the single heat exchanger 50. In one embodiment, theupper 53 and lower 56 portions of the tubular coil of the heat exchanger50 comprise approximately 1,300 ft. of 3″ seamless steel pipe with weldfittings.

Each row of the upper portion 53 of the heat exchanger 50 is constructedof a plurality of tubes 54 aligned in parallel with each other. Theoutlet of each tube 54 is connected in series with the inlet of anadjacent tube 54 by means of an approximate 180° curved tube or returnbend 55. Similarly, each planar row is connected in series to theadjacent rows above and below by connecting the outlet of the outermosttube in one row with the inlet of the outermost tube in another row bymeans of a return bend 55 a. In a preferred embodiment, each planar rowis laterally offset from the planar row above and below it so that thetubes 54 in one row are centered on the space between two adjacent tubes54 in the rows above and below it.

Each return bend 55 may further include an alignment bolt 47 extendingfrom the approximate exterior inflection point of the return bend 56.The multiple alignment bolts 47 correspond to holes formed in analignment plate 98, which is fixably attached to the upper portion 53 ofthe heat exchanger 50 by means of mechanical fasteners 45, such asthreaded nut fasteners. The alignment plate 98 maintains the alignmentof the stacked planar rows of the upper portion 53 of the heat exchanger50 so that the adjacent rows do not touch and space is maintainedbetween all adjacent tubes 54, thereby enabling the flow of heated airthrough the upper portion 53 of the heat exchanger 50 during operation.

The upper portion 53 is fluidly connected in series to the lower portion56 of the heat exchanger 50. As shown in FIGS. 4A-4B, the lower portion56 transitions to an angled rectangular helical coil configuration,which is oriented about a horizontal plane and defines a five-sidedcavity/chamber or tunnel 65. As will be described infra, the tunnel 65serves as an effective combustion chamber for the multiple oil-firedburner assemblies 60. The lower portion 53 of the heat exchanger 50comprises a tubular coil constructed a plurality of adjacently alignedupper 57 a and lower 57 b lateral tubes, which are vertically spaced andconnected in series by means of quarter-bend (i.e., approximately 90°bend) tubes 58 and riser tubes 59. The outlet of each lateral tube 57 isfluidly connected in series with the inlet of the next vertically spacedlateral tube 57 by means of a quarter-bend tube 58 followed by a risertube 59 followed by another quarter-bend tube 58. As shown in FIG. 4A,the outlet of the last lateral tube 57 in the tubular coil forming thelower portion 53 is fluidly connected to the outlet 52 of the heatexchanger 50.

With reference now to FIG. 4C, a cross-sectional view of the heatexchanger 50 shown in FIGS. 4A-4B installed in the firebox 40 of thepresent invention is shown. The firebox 40 includes a refractive lining48 configured between the interior walls and bottom of the firebox 40and the tubular coil of the heat exchanger 50. In a preferredembodiment, the firebox 40 may further include at least one vent 40 band passageway 40 c, which supplies ambient air from the upper exteriorof the firebox 40 to the front of the burner assemblies 60. Thepassageway 40 c is typically configured between the exterior wall of thefirebox housing 40 and the refractive lining 48. The vent 40 b andpassageway 40 c enable the burner assemblies 60 to operate with theaccess door 40 a configured in a “closed” position, which significantlyreduces the operational noise created by burner assemblies 60 whenoperating. Moreover, with the burner access doors 40 a configured in the“down” position, the danger inherent in a blowback event of the burnerassemblies 60 is greatly reduced.

As previously described, the heat exchanger 50 is comprised of a tubularcoil which is configured in a highly oscillating and serpentine mannerand oriented along two axes so as to maximize its exposure to the heatgenerated by the oil-fired burner assemblies 60. The upper portion 53configured in tightly stacked horizontal rows of tubing faked down in aseries of reversing loops oriented about a vertical axis; and a lowerportion 56 configured in a helical coil oriented about a horizontalaxis. The upper portion 53 is fluidly connected to the lower portion 56forming the single heat exchanger 50. The attached alignment plate 98maintains the alignment of the stacked planar rows of the upper portion53 of the heat exchanger 50 so that the adjacent rows do not touch andspace is maintained between all adjacent tubes 54, thereby enabling theflow of heated exhaust or flue gases 88 through the upper portion 53 ofthe heat exchanger 50 during operation. The lower portion 56 of the heatexchanger 50 transitions to an angled rectangular helical coilconfiguration, which is oriented about a horizontal plane and defines afive-sided cavity/chamber or tunnel 65.

The tunnel 65 serves as an effective combustion chamber for the multipleoil-fired burner assemblies 60 configured in the lower side of thefirebox 40. Each burner assembly 60 is connected to the fuel system anda pressurized air supply. For example, as schematically depicted inFIGS. 7 and 8, liquid fuel is supplied from the fuel tank 20 to eachburner assembly 60 via fuel pump 22, pressurized fuel line 26, fuelpressure control motor valve 27 and the metered pressurized fuel line28. Similarly, pressurized air for combustion is supplied to a primaryair inlet 62 configured on each burner assembly 60 via a primary airconduit 78 c. With reference again to FIG. 4C, the primary air and fuelare combined in the burner assembly 60 and directed through an atomizernozzle 64, which projects an atomized fuel spay F_(A) into the firebox40 where it is combusted in the previously described cavity/chamber ortunnel 65 formed in the heat exchanger 50. It is further noted that eachburner assembly 60 is oriented so as to initially generate asubstantially horizontal combustion flow 69 within the firebox 40. Eachburner assembly 60 includes self-contained controls 66 for adjusting thefuel-air mixture and an ignition mechanism for initially igniting thefuel-air mixture.

The firebox 40 depicted in FIGS. 4C, 7 and 8 further includes ductwork85 a, 85 b, which supply pressurized secondary air to the interior offirebox 40. The pressurized secondary air assists in directing andregulating the flow of heated flue gases 88 through the heat exchanger50 during operation. The ductwork 85 a, 85 b supplies pressurizedsecondary air to vents 86, 87 configured on opposing sides of thefirebox 40. The vents 86, 87 are typically configured so that theirrespective airflows F_(B), F_(C) are generally directed into thecavity/chamber or tunnel 65 formed in the heat exchanger 50. Thesecondary airflows F_(B), F_(C), which are projected from theirrespective vents 86, 87, assist in regulating and directing the flow ofheated flue gases 88 through the heat exchanger 50 during operation.

For example, a first or front vent 86 is configured under the burnerassemblies 60 and projects a first flow of secondary pressurized airF_(B) into the open front portion of the cavity/chamber or tunnel 65formed in the heat exchanger 50. In one embodiment, the first vent 86comprises an individual nozzle vent configured under each burnerassembly 60. The first flow of secondary pressurized air F_(B) providesa thermal air barrier that partially insulates the lateral tubes 57 b onthe bottom of the heat exchanger 50 from the substantially horizontalcombustion flame 69 generated by the burner assembly 60. In addition,the first flow of secondary pressurized air F_(B) absorbs the heatproduced by the substantially horizontal combustion flow 69 generating aflow of heated flue gases 88, which exhausts up through the heatexchanger 50 during operation. In a preferred embodiment, the first vent86 is angled at a slightly upward angle, so that the first flow ofsecondary pressurized air F_(B) combines with the atomized fuel spayF_(A) to effectively supercharge the resulting combustion flow 69 withadditional air.

The second or rear vent 87 is configured on the opposing wall or sidefrom the first vent 86 and burner assemblies 60, and projects a secondflow of secondary pressurized air F_(C) into the rear portion of thecavity/chamber or tunnel 65 formed in the heat exchanger 50. As depictedin Figures, the rear portion of the cavity/chamber or tunnel 65 formedin the heat exchanger 50 is partially obscured by the lateral tubes 57 ctraversing the tunnel 65. Thus, the second or rear vent 87 is configuredso as to project the second flow of secondary pressurized air F_(C)through gaps existing between adjacent lateral tubes 57. The injectionof the second flow of secondary pressurized air F_(C) provides a thermalair barrier that partially insulates the lateral tubes 57 c traversingthe back of the heat exchanger 50. In addition, the second flow ofsecondary pressurized air F_(C) also absorbs the heat produced by thesubstantially horizontal combustion flow 69 generating a flow of heatedflue gases 88, which exhausts up through the heat exchanger 50 duringoperation. In one embodiment, the second vent 87 may also be angled at aslightly upward angle.

Air Supply System

With reference again to the Figures, and in particular to FIGS. 5 and 6the air supply system of the present invention will be described ingreater detail. The air supply system of the present invention aforced-air or pressurized system which is not susceptible to changes inambient conditions, such as wind direction or gustiness. The air supplysystem of the present invention is comprised of primary and secondaryair systems. The primary air system supplies large volumes ofpressurized air to the multiple burner assemblies 60 configured in theside of the firebox 40. The primary air system includes a high-pressurepump which compresses ambient air and directs it to the primary airinlet 62 of each oil-fired burner assembly 60 where it is used toatomize fuel. The secondary air system supplies large volumes ofpressurized air to strategic locations within the firebox 40 to controland regulate the heating of the heat exchanger 50 and firebox 40. Thesecondary air system includes a secondary air blower mechanism, whichdraws in large volumes of ambient air. The secondary air is thendirected via ductwork to the previously described vents 86, 87configured on opposing sides of the firebox 40. The secondary airassists in maximizing the combustion of the fuel/air mixture whiledirecting and regulating the flow of heated flue gases 88 through theheat exchanger 50 during operation. By controlling and regulating theheating of the heat exchanger 50 and firebox 40 during operation, theoil-fired heat exchanger system 100 of the present invention cancontinuously heat large volumes of treatment fluid safely.

In the embodiment of the present invention 100 depicted in the Figures,the air supply system is comprised of matched sets of primary andsecondary blower systems disposed on opposing sides (i.e., the front andrear) of the firebox 40 in a mirror-image configuration. Each setincludes a primary blower system 70 and a secondary blower system 80,which are powered by a single motor mechanism. For example, the first orfront of blower system set is powered by motor 36 while the second orrear blower system set is powered by motor 37. The single motormechanism 36, 37 are preferably hydraulically powered. For example, inthe depicted embodiment, the motors 36, 37 are powered by hydraulicpumps 33, 34, respectively, which are driven by the accessory pump drivegear box 32. As noted previously, in a preferred embodiment, thehydraulic pumps 33, 34 comprise mechanically-driven hydraulic pumpswhich are rated at 5000 psi, but typically operate at approximately2500-3000 psi.

As shown in FIG. 5, which depicts in greater detail the second or rearblower system of the present invention 100, each primary air blowersystem 70 includes a high-pressure blower pump 74 having an intake whichdraws ambient air through an intake filter 72 and intake conduit 73. Ina preferred embodiment, each high-pressure blower pump 74 is a positivedisplacement rotary blower. Each high-pressure blower pump 74 is poweredby its respective motor mechanism 36, 37 through a rotary driveshaft 84.The high-pressure blower pump 74 compresses the air and directs it viaprimary air conduits 78 a, 78 b, 78 c to the primary air inlet 62 ofeach oil-fired burner assembly 60. The primary air conduits 78 a, 78 b,78 c may further include a primary air silencer 76, which muffles thenoise generated by the suction of ambient air into the primary airsystem 70. In one embodiment, the primary air conduits 78 a, 78 b, 78 calso include a pressure relief “pop-off” valve, which limits the primaryair pressure to approximately 5 psi.

Each secondary air system 80 includes one or more secondary air blowers81, which are also powered by the respective motor mechanism (e.g., 37)through a common rotary driveshaft 84. As shown in the FIG. 6, in oneembodiment the one or more secondary air blowers 81 each comprise aconventional centrifugal or squirrel-cage fan mechanism 82 contained ina protective housing 83. As depicted, the one or more fan mechanisms 82are aligned in a parallel configuration along and coupled to a commonrotary driveshaft 84 so that when the driveshaft 84 rotates, each fanmechanism 82 also rotates within its housing 83. It is further notedthat the co-alignment of the rotary shaft 84 with the fan mechanisms 82of the secondary air system 80 and the high-pressure blower pump 74 ofthe primary air blower system 70 enables both air supply systems to besimultaneously powered by the same motor 37.

The protective housing 83 of each secondary air blower 81 includes anopening, which allows the fan mechanism 82 to draw ambient air into itshousing 83 where it is directed to the ductwork of the secondary airsystem. The output of pressurized air from the secondary air blowers 81is combined in a first ductwork 85, which then divides into secondaryductwork 85 a, 85 b, which supply pressurized secondary air to vents 86,87 configured on opposing sides of the firebox 40. In the depictedembodiment, secondary air is pressurized to approximately 2.5-3 psi. Aspreviously noted, the vents 86, 87 are typically configured so thattheir respective airflows F_(B), F_(C) are generally directed into thecavity/chamber or tunnel 65 formed in the heat exchanger 50. Thesecondary airflows F_(B), F_(C), which are projected from theirrespective vents 86, 87, assist in regulating, directing, and enhancingthe convective flow of heated flue gases 88 through the heat exchanger50 during operation.

As shown in the embodiment depicted in FIG. 5, the first or front vents86 preferably comprise oblong circular vents positioned below thenozzles 64 of the burner assemblies 60. The depicted oblong circularvents 86 extend away from the firebox 40 wall and project one secondaryair stream F_(B) up towards the fuel/air mixture spray F_(A) generatedby the burner fuel nozzle 64. The second or rear vent 87 is configuredon the opposing wall of the firebox 40. As noted previously, theconfiguration of the second oblong circular vents 87 provides a layer ofcooling air F_(C) between the main burner fire and the bottom of thefirebox. Moreover, the angular set of the secondary vents 86, 87 causestheir respective opposing secondary air flows F_(B), F_(C) to collide inthe tunnel 65 formed in the heat exchanger 50, thereby affecting theflow of heated exhaust or flue gases 88 up and through the upper portion53 of the heat exchanger 50 during operation.

The integrated temperature controller mechanism 68 in conjunction withforced-air supply system and refractive insulation lining 48 in thefirebox 40 enable the oil-fired heat exchanger system 100 of the presentinvention to safely heat water continuously. Operation time is limitedonly by fuel supply. For example, the depicted embodiment of the presentinvention 100, which is configured with six (6) burner assemblies 60,typically consumes 150-165 gallons of fuel per hour. The burner fueltank 20 on the unit holds about 2500 gallons and is therefore sized for15-16.5 hours of continuous operation. The auxiliary powerplant 30 hasits own fuel tank that holds approximately 150 gallons of fuel thatallow it to operate up to 18 hours depending on operating conditions. Inthe field, operators may have additional fuel delivered every 12 hoursor so to allow the system 100 to continue operations on large heatingjobs.

Accessory Fuel Oil Heating System

With reference now to FIG. 9, a schematic depiction of an embodiment ofan Accessory Fuel Oil Heating System is shown. As demonstrated in thedepiction, the accessory fuel oil heating system of the presentinvention may be an integral part of the frac water heater system 100 ofthe present invention or comprise a separate, stand-alone unit, whichmay be selectively attached to any oil-fired frac water heating system.The accessory fuel oil heating system of the present invention includesan accessory fuel oil heat exchanger unit 110 having a manifold 112 forreceiving and distributing the fuel oil through a plurality of heatexchanger tubes, and directing it to a heated fuel line 126, whichsupplies the burners 60 with heated fuel oil for combustion. A returnline 25 a may also be installed to allow a percentage of the heated fueloil to return back to the fuel tank 20 to preheat the fuel oil in thefuel tank 20. The unit 110 further includes an inlet 114 for receivingheated treatment fluid from the frac water heater system of the presentinvention. The unit 110 is typically configured (either temporarily orpermanently) along the outlet conduit 95 between the outlet 52 of themain heat exchanger 50 and the outlet manifold 96. The unit 110 includesan inlet 114, which is fluidly connected to outlet conduit 95, and anoutlet 118 fluidly connected to the outlet manifold 96. Thus, as theheated treatment fluid from the frac water heater system passes throughthe accessory fuel oil heat exchanger unit 110, it also passes over theplurality of heat exchanger tubes in the accessory heat exchanger unit110 prior to being directed to the outlet 118 and on to the outletmanifold. As the heated treatment fluid passes over the plurality ofheat exchanger tubes, heat energy is transferred from the treatmentfluid to the fuel oil raising its temperature significantly.

The accessory fuel oil heating system of the present invention receivesfuel oil from fuel oil inlet line 125 which is fluidly connected to thefuel tank 20 via the standard unpressurized fuel line 21, fuel pump 22and pressurized fuel line 26. The unpressurized fuel line 21 may alsoinclude a shut-off valve 21 a to isolate the fuel tank 20 from thesystem during transit and for maintenance. An isolation valve 121 isconfigured between the oil inlet line 125 and the pressurized fuel line26. The isolation valve 121 may be closed to bypass the accessory fueloil heat exchanger unit 110 when weather conditions to do not demand itsuse. In such instances, by-pass valve 123 is opened to by-pass theaccessory fuel oil heat exchanger unit 110 allowing the pressurized fuelin pressurized fuel line 26 to flow directly to the burners 60.

However, during operation of the accessory fuel oil heating system ofthe present invention, the isolation valve 121 is opened and the by-passvalve 123 is closed allowing fuel oil in pressurized fuel line 26 toflow directly through the fuel oil inlet line 125 and into the inletmanifold 112 of the accessory fuel oil heat exchanger unit 110. As willbe explained in greater detail below, the fuel oil flows through theaccessory fuel oil heat exchanger unit 110, where it is heated, andexits the fuel outlet 140 which is fluidly connected to the heated fueldischarge line 126. By heating the fuel oil prior to combustion, theaccessory fuel oil heating system of the present invention greatlyimproves the viscosity and flow rate of the fuel oil, thereby allowingcheaper, less cold-tolerant grades of fuel oil to be used in extremecold climates with no degradation in the operation of the system of thepresent invention.

The accessory fuel oil heating system of the present invention may alsoinclude a fuel filter 23, which filters the fuel oil prior to directingit to the burners 60 in much the same way as the standard system of theinvention. The fuel filter 23 is configured downstream of the fuelrelief line 25 a, pressure relief valve 24 and fuel return line 25. Thepressure relief valve 25 maintains proper fuel pressure by relievingexcess fuel oil back to the fuel tank 20. Moreover, in the event thefuel filter 23 becomes clogged, the pressure relief valve 24 permitsexcess fuel oil to flow back to the fuel tank 20 via fuel return line 25a, 25.

The accessory fuel oil heating system of the present invention may alsoinclude a solenoid operated safety valve 26 a. In the event of amalfunction, the safety valve 26 a will lose power from the controlpanel causing it to close and shutting off fuel supply to the burners.The accessory fuel oil heating system of the present invention utilizesthe same fuel pressure control motor valve 27 as previously disclosed.However, the system also includes a pilot valve 27 a configured aroundthe fuel pressure control motor valve 27. The pilot valve 27 a permitsan operator to allow a small amount of fuel to bypass the fuel pressurecontrol motor valve 27 and keep the burners lit when the fuel pressurecontrol motor valve 27 is closed.

With reference now to FIGS. 10, 11A-B, and 12, an embodiment of theaccessory fuel oil heat exchanger unit 110 is depicted. The accessoryfuel oil heat exchanger unit 110 comprises a heat exchanger manifoldassembly 120 coaxially aligned with an outer housing unit 116. Thehousing unit 116 includes a flange 117 at its nominal end for couplingwith a matching flange 115 formed on the manifold assembly 120. In apreferred embodiment, the two flanges 115, 117 are fastened togetherwith bolt fasteners. A flexible seal configured between the two flanges115, 117 is typically used to ensure a water-tight seal.

The heat exchanger manifold assembly 120 includes a manifold 112 forreceiving and distributing the fuel oil through a plurality of heatexchanger tubes, and directing it to a fuel outlet 140 that is fluidlyconnected to the heated fuel line 126. The assembly 120 further includesan inlet 114 for receiving heated treatment fluid from the main heatexchanger 50 of the frac water heater system 100 of the presentinvention.

As best illustrated in FIGS. 13A and B, the manifold 112 includes awater inlet passageway 122 for receiving heated treatment fluid. Thepassageway 122 is configured in the center of the manifold 112 and isfluidly connected to the inlet conduit 114. The manifold 112 furthercomprises two annular walls 136, 138, which are coaxially aligned withthe passageway 122. The annulus between the inner 136 and outer 138walls is divided into two chambers by two divider walls 137. The twochambers 132, 142 are enclosed by annular plates 124 fixably attached tothe opposing edges of walls 136, 137, 138. For example, in a preferredembodiment the annular plates 124 are welded to the opposing edges ofwalls 136, 137, 138.

The manifold 112 further includes an inlet 130 which fluidly couples thefuel oil inlet line 125 to the first or inlet chamber 132. The innerwall 136 of the inlet chamber 132 includes openings, which fluidlyconnect the inlet chamber 132 to a plurality of inlets of heat exchangetubes. For example, in the depicted embodiment the inner wall 136includes four openings for receiving inlets 152, 162, 172, 182 for fourheat exchange tubes.

Correspondingly, the manifold 112 also includes an outlet 140 whichfluidly couples the second or outlet chamber 142 to the heated fueldischarge line 126. The inner wall 136 of the outlet chamber 132includes openings, which fluidly connect the outlet chamber 142 to aplurality of outlets of heat exchange tubes. For example, in thedepicted embodiment the inner wall 136 includes four openings forreceiving outlets 154, 164, 174, 184 for four heat exchange tubes.

Thus, the inlet chamber 132 is fluidly connected to the outlet chamber142 by means of a plurality of heat exchange tubes connected tocorresponding inlets and outlets.

The heat exchange tubes comprise U-shaped tubes having a single inletand a single outlet. The U-shaped tubes have substantially the samelateral width but are of varying length in order to allow the pluralityof heat exchange tubes to nest within each while minimizing overallobstruction. For example, in the embodiment depicted in FIGS. 10, 11A-B,and 12, the heat exchanger manifold assembly 120 includes a firstU-shaped heat exchange tube 150 having an inlet 152 fluidly coupled tothe inlet chamber 132 and an outlet 154 fluidly coupled to the outletchamber 142. Likewise, the assembly 120 further includes a secondU-shaped heat exchange tube 160 having an inlet 162 fluidly coupled tothe inlet chamber 132 and an outlet 164 fluidly coupled to the outletchamber 142. The length of the second heat exchange tube 160 is greaterthan the length of the first heat exchange tube 150 which allows theU-shaped turn 156 of the first heat exchange tube 150 to easily nestbetween the second heat exchange tube 160 with minimal obstruction.Similarly, the assembly 120 further includes a third U-shaped heatexchange tube 170 having an inlet 172 fluidly coupled to the inletchamber 132 and an outlet 174 fluidly coupled to the outlet chamber 142.The length of the third heat exchange tube 170 is greater than thelength of the second heat exchange tube 160 which allows the U-shapedturn 166 of the second heat exchange tube 160 to easily nest between thethird heat exchange tube 170 with minimal obstruction. Finally, theassembly 120 also includes a fourth U-shaped heat exchange tube 180having an inlet 182 fluidly coupled to the inlet chamber 132 and anoutlet 184 fluidly coupled to the outlet chamber 142. The length of thefourth heat exchange tube 180 is greater than the length of the thirdheat exchange tube 170 which allows the U-shaped turn 176 of the thirdheat exchange tube 170 to easily nest between the fourth heat exchangetube 180 with minimal obstruction.

The inlet chamber 132 and outlet chamber 142 may each further comprise apressure outlet for receiving a pressure gauge. For example, the inletchamber 132 may further include an outlet 134 for receiving a pressuregauge 135. Similarly, the outlet chamber 142 may further include anoutlet 144 for receiving a pressure gauge 145. The pressure gauges maybe simple sight gauges as depicted or may comprise electronic gaugeslinked to a central control panel.

As shown in the Figures, and in particular FIGS. 14A-B, the heatexchanger manifold assembly 120 may also include one or more supportbrackets 194 configured around the plurality of heat exchange tubes. Thesupport bracket 194 assists in maintaining the spacing and alignment ofthe various heat exchange tubes, both in relation to one another, and inrelation to the outer housing unit 116. The support bracket 194 preventsthe various heat exchange tubes (e.g., 150, 160, 170, 180) fromcontacting the inner wall of the outer housing unit 116. In a preferredembodiment, the support bracket 194 comprises a plurality of rings 197arranged in an annular configuration and connected by linking bars 198between adjacent rings. The rings 197 have an inner diameter 196 whichis greater than the diameter of the various heat exchange tubes. Themaximum diameter 197 of the support bracket 194 is less than theinterior diameter of the outer housing unit 116.

The outer housing unit 116 of the accessory fuel oil heat exchanger unit110 includes an inlet of sufficient diameter to receive the plurality ofheat exchange tubes (e.g., 150, 160, 170, 180) extending from the heatexchanger manifold assembly 120. The previously described one or moresupport brackets 194 assist in preventing the various heat exchangetubes from contacting the inner wall of the outer housing unit 116. Theouter housing unit 116 also includes an outlet 118 at its distal end fordischarging the heated treatment fluid. The outlet 118 is fluidlyconnected to the outlet manifold 96 of the overall system 100. Forexample, the outlet 118 may include screw threads 119 for fluidlyconnecting to the outlet manifold 96. It is important to note that thediameter 192 of the housing unit 116 at its nominal end is typicallygreater than the diameter 190 of the housing unit 116 at its distal end.In a preferred embodiment, the diameter 192 of the housing unit 116 atits nominal end is almost twice the diameter 190 of the housing unit 116at its distal end (6⅝ inches compared to 3½ inches). The diameter of theinlet 114 is substantially similar, preferably the same, as the diameter190 of the outlet 118. This difference in diameters of the outer housingunit 116 is critical to ensuring a smooth flow of the heated treatmentfluid through the accessory fuel oil heat exchanger unit 110 whilepreventing the inducement of back pressure on the main heat exchanger50.

Method of Operation

The system 100 of the present invention includes novel methods forheating large volumes of treatment fluid in a continuously flowingfashion so that on-site heating operations can be performed“on-the-fly”, i.e., without the use of preheated stockpiles of treatmentfluid. For example, the embodiment of the system 100 of the presentinvention depicted in the Figures, is capable of heating sufficientquantities of continuously flowing water to conduct “on-the-fly”hydraulic fracturing operations at remote well sites. The system 100 ofthe present invention also includes novel methods for controlling theheating of the treatment fluid as it passes through the system 100. Thesystem 100 of the present invention further includes novel methods forcontrolling the temperature change and volume flow of treatment fluid asit passes through the system 100.

With reference again to the Figures and in particular FIGS. 7 and 8, themethod of the present invention is depicted. A treatment fluid, such aswater, is drawn from an ambient fluid source into the system 100. Thetreatment fluid is then pumped through a single pass of a tubular coilheat exchanger 50 contained within firebox 40 where it is heated. As thetreatment fluid proceeds through the heat exchanger 50 it increases intemperature until it reaches the outlet 52 of the heat exchanger 50where it is directed via tubular conduits or hose to the well head forinjection into the formation.

The main fluid pump 94 is used to control the flow rate of the treatmentfluid through the system 100. For example, a supply hose (not shown)extending to the fluid source is connected to the intake manifold 90 soas to put the system 100 in fluid communication with the fluid source.The main fluid pump 94 draws the treatment fluid via conduits 93 a, 93 bfrom the fluid source and supplies it to the inlet 51 of the heatexchanger 50. The main fluid pump 94 has sufficient power to both drawthe treatment fluid from the fluid source and pump the treatment fluidthrough the heat exchanger 50 and on to the well head for injection intothe formation.

For example, in one embodiment, the main fluid pump 94 is capable ofsupplying treatment fluid to the heat exchanger 50 at a pressure ofabout 150 psi. In a preferred embodiment, the main fluid pump 94 is alsocapable of drawing and pumping a maximum of 252 gpm of treatment fluidthrough the system 100. The requisite volumetric flow rate of treatmentfluid is typically dictated by the particular operational requirementsdesired at the well head. By adjusting the speed of the main fluid pump94, the volumetric flow rate of treatment fluid is controlled. The mainfluid pump 94 is driven by a hydraulic motor 38 powered via supply line35 a by a hydraulic pump 35 attached to the accessory pump drive gearbox 32. Consequently, the speed of the main fluid pump 94 is controlledby the operator using a control lever 12 to increase or decrease theamount of pressurized hydraulic fluid supplied to hydraulic motor 38. Ina preferred embodiment, control lever 12 comprises an electronicjoystick actuator, which regulates the displacement of the hydraulicpump to change the speed of its respective hydraulic motor. Thehydraulic pressure depends on the loads placed on the hydraulic motors.

As the treatment fluid is pumped through the heat exchanger 50 containedwithin the firebox 40, the fluid is heated by the transfer of thermalenergy generated by the combustion of a liquid-fuel/air mixture in thefirebox 40. As previously detailed, pressurized primary air and liquidfuel are combined in the multiple burner assemblies 60, which eachproject an atomized fuel spay F_(A) into the firebox 40 where it iscombusted. The burner assemblies 60 are configured near the bottom ofthe firebox 40 and oriented so as to initially generate a substantiallyhorizontal combustion flow 69 within the firebox 40. Pressurizedsecondary air assists in directing and controlling the thermal energygenerated by the substantially horizontal combustion flow 69 to exhaustin a convective flow up and through the upper portion 53 of the heatexchanger 50.

The tubular coil heat exchanger 50 is designed to maximize the heattransfer of the thermal energy within the confines of the firebox 40.The heat exchanger 50 is, therefore, comprised of a tubular coil whichis configured in a two interconnected portions, which are oriented alongtwo distinct axes so as to maximize exposure to the heat generated bythe oil-fired burner assemblies 60. The ambient or cool treatment fluidenters the heat exchanger 50 through the inlet 51 configured at or nearthe top of the heat exchanger coil 50. As the fluid flows through theupper portion 53 of the heat exchanger 50 thermal energy is transferredby the convective flow of the hot flue gases 88 over and between thestacked horizontal rows of interconnected adjacent tubes faked down in aseries of reversing loops oriented about a vertical axis. As the fluidcontinues through the lower portion 56 of the heat exchanger 50 it flowsthrough a helical coil oriented about a horizontal axis, thermal energyis transferred by the both the convective flow of the hot flue gases 88and the radiant heat emanating from the substantially horizontalcombustion flow 69 within the cavity/chamber or tunnel 65.

The convective flow of flue gases 88 through heat exchanger 50 issubstantially enhanced by the secondary air system, which continuallysupplies large volumes of pressurized air to strategically configuredvents 86, 87 on opposing sides of the firebox 40. The secondary air flowis essentially a forced air system which uses air as its heat transfermedium to extract thermal energy from the substantially horizontalcombustion flow 69. The vents 86, 87 are positioned near the bottom ofthe closed-bottom firebox 40 and configured so that their respectiveairflows F_(B), F_(C) are generally directed into the cavity/chamber ortunnel 65 formed in the heat exchanger 50.

The treatment fluid continues to absorb thermal energy as it flowsthrough the lower portion 56 of the heat exchanger 50 until it reachesthe outlet 52 of the heat exchanger 50 where it is directed via tubular95 and supply hose (not shown) to the well head for injection into theformation.

As the heated treatment fluid exits the outlet 52 of the heat exchanger50 its temperature is monitored. The temperature of the treatment fluidexiting the heat exchanger outlet 52 is a function of three variables:the volumetric flow rate of the treatment fluid through the heatexchanger 50; the flow rate of the pressurized secondary air; and theheat generated by the multiple burner assemblies 60 configured in theheat exchanger 50. The flow rate of the secondary air is typically heldconstant during all operations while the volumetric flow rate of thetreatment fluid is typically constant for a given operation. Thus, thetemperature of the treatment fluid exiting the heat exchanger outlet 52is controlled by regulating the volume of fuel supplied to the multipleburner assemblies 60.

In one embodiment, the operator monitors the temperature of the heatedtreatment fluid as it exits the outlet 52 of the heat exchanger 50. Theoperator then adjusts the temperature controller mechanism 68 sending acontrol signal to the fuel pressure control motor valve 27 to increaseor decrease the volume of fuel supplied to the multiple burnerassemblies 60 via pressurized metered fuel lines 28. The control signalmay comprise an electrical, wireless, pneumatic, or hydraulic signal.For example, in the depicted embodiment, the adjustable temperaturecontroller mechanism 68 comprises a simple manual rotary valve, whichcontrols the pneumatic pressure supplied to the fuel pressure controlmotor valve 27.

In another embodiment, the temperature controller mechanism 68 is anautomated thermostat mechanism that continually monitors the temperatureof the treatment fluid exiting the heat exchanger outlet 52. An operatorinputs a desired temperature reading (i.e., set point temperature). Thetemperature controller mechanism 68 compares the actual temperature ofthe treatment fluid exiting the heat exchanger outlet 52 with the setpoint temperature and automatically adjusts the control signal suppliedto the fuel pressure control motor valve 27. For example, if thetemperature of the treatment fluid exiting the heat exchanger outlet 52is less than the set point temperature, the temperature controllermechanism 68 adjusts the control signal supplied to the fuel pressurecontrol motor valve 27 to increase the volume of fuel supplied to themultiple burner assemblies 60 via pressurized metered fuel lines 28 inorder to maintain a set point temperature. Conversely, if thetemperature of the treatment fluid exiting the heat exchanger outlet 52is higher than the set point temperature, the temperature controllermechanism 68 adjusts the control signal supplied to the fuel pressurecontrol motor valve 27 to decrease the volume of fuel supplied to themultiple burner assemblies 60 via pressurized metered fuel lines 28 inorder to maintain a set point temperature.

The temperature of the treatment fluid is also typically monitored atthe inlet 51 of the heat exchanger 50. The temperature spread betweenthe inlet 51 and outlet 52 of the heat exchanger 50, when combined withthe volumetric flow rate of treatment fluid, is indicative of theheating capacity of the system. Field testing has determined that thedepicted embodiment of the oil-fired heat exchanger system 100 of thepresent invention is capable of heating ambient water from 70° F. to210° F. at a maximum volumetric flow rate of 252 gpm. Moreover, fieldreports further indicate that the system 100 is capable of heating waterfrom 40° F. to 210° F. in ambient atmospheric temperatures below 25° F.at a slightly reduced volumetric flow rate (e.g., 200-250 gpm).

Method of Operation of Accessory Fuel Oil Heating System

When not needed, the accessory fuel oil heating system of the presentinvention may be isolated from the frac water heating system 100 of thepresent invention. For example, as shown in FIG. 9 the isolation valve121 is closed in such instances and the by-pass valve 123 is openedallowing fuel oil in the pressurized fuel line 26 to by-pass theaccessory fuel oil heat exchanger unit 110 and proceed directly toburners in accordance with the standard operating procedure discussedpreviously. In such cases, the heated treatment fluid still proceedsthrough the outlet conduit 95 and the accessory fuel oil heat exchangerunit 110, but no heat transfer to the fuel oil occurs in the accessoryfuel oil heat exchanger unit 110.

When the weather conditions warrant utilization of the accessory fueloil heating system of the present invention the isolation valve 121 isopened and the by-pass valve 123 is closed allowing fuel oil inpressurized fuel line 26 to flow directly through the fuel oil inletline 125 and into the inlet manifold 112 of the accessory fuel oil heatexchanger unit 110. The inlet manifold 112 receives the fuel oil viainlet 130, which directs the fuel oil into the inlet chamber 132 whereit is distributed to the inlets (e.g., 152, 162, 172, 182) of theplurality of heat exchange tubes (e.g., 150, 160, 170, 180). Heatedtreatment fluid from the frac water heater system 100 is directedthrough the accessory fuel oil heat exchanger unit 110 where it passesover the plurality of heat exchanger tubes prior to being directed tothe outlet 118 and on to the outlet manifold 96. As the heated treatmentfluid passes over the plurality of heat exchanger tubes, heat energy istransferred from the treatment fluid to the fuel oil raising itstemperature significantly.

Each of the plurality of heat exchanger tubes (e.g., 150, 160, 170, 180)direct the fuel oil to its respective outlet (e.g., 154, 164, 174, 184)where it is discharged into the outlet chamber 142. The outlet chamber142 directs the heated oil to the heated fuel discharge line 126 via thefuel outlet 140. The heated fuel oil then proceeds via heated fuel line126 to the burners 60 in substantially the same manner as previouslydescribed in the method of operation of the system 100 of the presentinvention.

In the event that the accessory fuel oil heat exchanger unit 110produces more heated oil than required at the moment by the burners, thesystem of the present invention will direct the excess fuel oil back tothe fuel tank 20 via fuel return line 25 a, 25 when the pressure in fuelline 126 triggers the pressure relief valve 24.

By heating the fuel oil prior to combustion, the accessory fuel oilheating system of the present invention greatly improves the viscosityand flow rate of the fuel oil, thereby allowing cheaper, lesscold-tolerant grades of fuel oil to be used in extreme cold climateswith no degradation in the operation of the system of the presentinvention. While the heat energy extracted from the treatment fluid hasa negligible effect on the overall temperature of the treatment fluid,it significantly affects the temperature of the fuel oil discharged bythe system. Although diesel fuel is typically utilized as the preferredform of fuel oil in the present invention, it is understood that theaccessory fuel oil heating system of the present invention is applicableto a wide variety of fuel oils and significantly improves the combustionand flow characteristics of many fuel oils, which would otherwise beimpractical to use.

It will now be evident to those skilled in the art that there has beendescribed herein an improved heat exchanger system for heating large,continuously flowing volumes of treatment fluids at remote locations.Although the invention hereof has been described by way of a preferredembodiment, it will be evident that other adaptations and modificationscan be employed without departing from the spirit and scope thereof. Forexample, instead of the treatment fluid being water, it could be apetroleum based liquid such as oil for hot oil well treatments. Theterms and expressions employed herein have been used as terms ofdescription and not of limitation; and thus, there is no intent ofexcluding equivalents, but on the contrary it is intended to cover anyand all equivalents that may be employed without departing from thespirit and scope of the invention.

I claim:
 1. An accessory fuel oil heating system for a frac-water heating system, comprising: a heat exchanger unit having a manifold for receiving and distributing fuel oil through a plurality of heat exchanger tubes prior to directing said fuel oil to the burners of the frac-water heating system for combustion, said heat exchanger unit having an inlet for receiving heated treatment fluid from the frac-water heating system and directing the heated treatment fluid over the plurality of heat exchanger tubes prior to directing the heated treatment fluid to an outlet of said heat exchanger unit.
 2. The accessory fuel oil heating system of claim 1, wherein the heat exchanger unit comprises a heat exchanger manifold assembly coaxially aligned with an outer housing unit.
 3. The accessory fuel oil heating system of claim 2, further comprising at least support bracket configured around the plurality of heat exchanger tubes, wherein the support bracket maintain the spacing and alignment of the heat exchanger tubes and prevent the heat exchanger tubes from contacting the outer housing unit.
 4. The accessory fuel oil heating system of claim 2, wherein the outer housing unit includes an inlet having a diameter almost twice as large as the diameter of the outlet.
 5. The accessory fuel oil heating system of claim 1, wherein the manifold includes a passageway which extends through the manifold for receiving the heated treatment fluid.
 6. The accessory fuel oil heating system of claim 1, wherein said manifold comprises an inlet chamber fluidly connected to an outlet chamber by the plurality of heat exchanger tubes.
 7. The accessory fuel oil heating system of claim 6, wherein the inlet chamber is fluidly connected to a fuel tank via a fuel pump and pressurized fuel line.
 8. The accessory fuel oil heating system of claim 7, wherein the pressurized fuel line includes an isolation valve and a by-pass valve for bypassing heat exchanger unit when not needed.
 9. The accessory fuel oil heating system of claim 1, further comprising a pressure relief valve configured downstream of the heat exchanger unit to allow excess heated fuel oil to be directed back into the fuel tank.
 10. The accessory fuel oil heating system of claim 1, wherein the plurality of heat exchanger tubes are U-shaped and each has a different length.
 11. The accessory fuel oil heating system of claim 1, wherein the inlet of said heat exchanger unit is fluidly connected to an outlet conduit of the frac-water heating system.
 12. The accessory fuel oil heating system of claim 1, wherein the outlet of said heat exchanger unit is fluidly connected to an outlet manifold of the frac-water heating system.
 13. The method for heating fuel oil prior to combustion in a frac-water heating system, comprising in combination: directing fuel oil to a manifold of a heat exchanger unit, wherein said manifold receives and distributes said fuel oil through a plurality of heat exchanger tubes prior to directing said fuel oil to the burners of the frac-water heating system for combustion, supplying an inlet in the heat exchanger unit with heated treatment fluid from the frac-water heating system; directing the heated treatment fluid over the plurality of heat exchanger tubes prior to directing the heated treatment fluid to an outlet of said heat exchanger unit.
 14. The method for heating fuel oil of claim 13, wherein the step of supplying the inlet of the heat exchanger with heated treatment fluid comprises fluidly connecting the inlet of the heat exchanger unit to an outlet conduit of the frac-water heating system.
 15. The method for heating fuel oil of claim 13, wherein the heated treatment fluid passes over the plurality of heat exchanger tubes in a single pass.
 16. The method for heating fuel oil of claim 13, wherein said manifold distributes the fuel oil through the plurality of heat exchanger tubes by receiving fuel oil in an inlet chamber that is fluidly connected to an outlet chamber by the plurality of heat exchanger tubes.
 17. The method for heating fuel oil of claim 16, wherein the step of directing fuel oil to the manifold of the heat exchanger unit comprises fluidly connecting the inlet chamber to a fuel tank via a fuel pump and pressurized fuel line.
 18. The method for heating fuel oil of claim 17, further comprising allowing excess heated fuel oil to be directed back into the fuel tank by configuring a pressure relief valve downstream of the heat exchanger unit.
 19. The method for heating fuel oil of claim 13, further comprising fluidly connecting the outlet of the heat exchanger unit to an outlet manifold of the frac-water heating system to direct heated treatment fluid to a wellbore application.
 20. The method for heating fuel oil of claim 13, wherein said treatment fluid is heated from 40° F. to 210° F. in ambient atmospheric temperatures below 25° F. while pumping treatment fluid through said heat exchanger unit at a volumetric flow rate ranging from 200-250 gpm. 